1. Field of the Invention
The present invention relates generally to a system and method for pointing the bore sight of a non-tracking antenna towards the center of a satellite station-keeping box. More particularly, by pointing the bore sight towards the center of the satellite station-keeping box, the largest excursion between the antenna""s bore sight and the actual satellite position is no greater than half the diagonal of the satellite station-keeping box.
2. Brief Description of the Related Art
Typically, installing a non-tracking satellite terminal (ST) antenna in a geo-synchronous satellite system requires knowledge of the desired satellite location on the geo-synchronous orbital arc and its elevation and azimuth angles relative to the ST location. The installer performs an initial search by pointing the antenna toward the direction of the target satellite until sufficient satellite signal power is received to declare the successful acquisition of the target satellite. Once the target satellite is acquired, the installer first performs a coarse antenna pointing optimization by peaking the strength of the received signal and noting this position at the antenna pointing mechanism. The installer then uses the bracketing technique to align the antenna bore sight to the satellite with fine adjustments in elevation and azimuth. The bracketing technique entails having the installer mechanically dither the antenna pointing in such a manner that, when the antenna bore sight is pointed at the satellite, the signal strength at a fixed amount away from the bore sight in azimuth/elevation registers the same measured amount. This is based on the fact that the antenna gain rolloff (in dB) can be approximated by a parabola both in azimuth and in elevation relative to the antenna bore sight.
Employing the mechanical bracketing method for fine elevation adjustment, the installer points the antenna away from the initially noted position on the pointing mechanism on either side until the measured signal strength is xcex1 dB less (e.g., usually xcex1=3 for C and Ku band terminals; whereas for Ka band terminal, xcex1=6 may be used). The installer records the amount of adjustments on both sides of this noted position on the antenna pointing mechanism for achieving the xcex1 dB less signal strength. If the amount of adjustment is not the same, then the installer adjusts the antenna pointing in the direction of the lower signal strength reading. The installer repeats this fine elevation adjustment process or dithering until the adjustments on both sides are exactly the same. At this point the fine elevation adjustment process is complete and the elevation of the bore sight is considered to be aligned to the satellite. Then, this fine elevation adjustment process is repeated for fine azimuth adjustment until the fine azimuth adjustment is completed. At this point the antenna is left untouched for operational use despite the fact that the satellite will eventually move away from its current position.
A satellite at its assigned geo-synchronous orbital location is not really stationary relative to the earth""s center fixed coordinate. This is due to the fact that the earth is not really an ideal sphere. In fact, it is more like an ellipsoid. As a consequence the satellite will move in altitude, azimuth and elevation relative to any fixed ST terminal location on the earth surface even though it is supposed to rotate synchronously with the earth""s rotation around its own axis. Station keeping thrusters must be used to keep the satellite within a box around its assigned orbital location. The supposedly stationary satellite actually moves around any where inside the 3-dimensional (Az, El, and ALT) cube. However, from the ST antenna pointing perspective, the altitude of the satellite is of less importance when compared to azimuth and elevation. So, for ST antenna pointing, the satellite position within the AZ and EL 2-dimensional box during and after installation is of importance. The ST antenna gain pattern around its bore sight rolls off approximately in Gaussian shape (and parabolic in dB), less signal strength is received by the satellite when it moves away from the ST antenna bore sight. Similarly, the ST receives less power from satellite transmisson when the satellite drifts away from the bore sight of the ST receiving antenna. At the time of installation, the actual satellite position can be any where inside the (Az, EL) station keeping box. It may be at a corner of the satellite station-keeping box, and at a later time the satellite may drift to the opposite corner. In this case, loss of signal power would be the largest, since the angular distance between the terminal antenna bore-sight and the actual satellite position would be the greatest, in the amount of the full diagonal of the satellite station-keeping box. If the actual satellite position at the time of installation is some where else inside the box, the loss in signal power is less.
To compensate for such signal power loss due to satellite motion and ST initial antenna pointing, margins must be built into the system designed to offset the antenna gain reductions due to non-perfect satellite station keeping, by using a higher terminal transmit EIRP (Effective Isotropic Radiated Power) and receive G/T. Both of which would have the undesirable consequences of requiring larger antenna size, higher amplifier power, lower receiver noise temperature, and ultimately higher terminal costs. The required margins for the conventional means of terminal antenna pointing are dictated by the worst-case scenario as cited above. That is, the satellite is at one corner of the box at the time of terminal installation when the antenna bore sight is aligned to the actual satellite position. At some later time the satellite moves away to the far corner.
On the other hand, it is desirable to align the ST terminal antenna bore sight to the center of the box regardless of the actual satellite position at the installation time, then the maximum signal loss can be drastically reduced, particularly for large non-tracking terminals. This is accomplished by taking advantage of the fact that the satellite position relative to the terminal bore sight can never exceed half the diagonal distance of the box regardless where the satellite is inside the box at any future time. Therefore, a need exists for an antenna pointing system and method wherein the antenna terminal is initially pointed toward the satellite station-keeping box center, thus allowing the largest error between the terminal antenna bore-sight and the actual satellite position to be no more than one half the diagonal of the satellite station-keeping box.
In addition, in conventional antenna positioning systems, after an installer has completed the tedious pointing exercise, he often discovers he has acquired the wrong satellite. This is due to the fact that the identification information of the satellite is obtained after completion of the additional tedious commissioning process for the terminal over and above what has already been a complicated antenna pointing exercise. As a consequence, the installer often times needs to redo the tedious antenna pointing process again and again until he has acquired the right satellite. Accordingly, it is highly desirable to have the ability to know whether the acquired satellite is the target satellite, without the need to go through the tedious terminal commissioning process. Therefore, a need exists for the use of satellite specific unique words to provide satellite identity without going through the tedious commissioning process. That is, once a satellite is acquired through the antenna pointing process, it should be almost certain that the true target satellite is acquired, and not any other neighboring satellites. The probability of acquiring the wrong satellite should be practically nil, say less than 10xe2x88x9210. The conventional method does not have such a capability.
In addition, the integrity of any antenna pointing method or system is judged by its sensitivity to five major sources of antenna pointing errors comprising:
(1) the inaccuracy of measured signal strength,
(2) the signal distortion caused by rain attenuation and atmospheric scintillation,
(3) the inaccuracy of the antenna pointing mechanism itself,
(4) the inaccuracy of the antenna pointing by the installer, and
(5) the effect of satellite movement within its station-keeping box.
Conventional methods employ the actual signal power to indicate signal strength which can vary widely from equipment to equipment and also can vary with the propagation condition of the satellite-to-terminal path particularly at Ka band. The resulting error signal necessary for antenna pointing control can be quite inaccurate, thereby rendering accurate antenna pointing impractical and in some cases impossible. The errors caused by items (3) through (5) above, while important, are common to the current method and hence will not be discussed in greater details.
The above described disadvantages are substantially overcome and advantages realized by a system and method for positioning an antenna adapted to receive a signal from at least one satellite in a communications network. The satellite position area in which the satellite is expected to be present is called the satellite station-keeping box which is divided into a plurality of regions. A satellite terminal coupled to the antenna is adapted to compute the distance measurement between the actual satellite position and a central position within the satellite station-keeping box. Upon computing the distance measurement the antenna is further adapted to point toward the central location of the satellite position area thereby decreasing the error despite the satellite movement within the satellite station-keeping box.
The above disadvantages are also substantially overcome by providing a method for positioning a satellite terminal antenna adapted to receive a signal from the satellite in a communications network. This method comprises identifying a satellite position area in which the satellite is expected to be present, and dividing that area into a plurality of regions. Then, identifying a first signal, at a first location by pointing from the satellite terminal antenna toward the satellite located within one of the regions of the satellite position area. Then, computing a satellite position error by determining a difference between the first location and a point located substantially central to the satellite position area. Finally, adjusting the first signal by an amount of the satellite position error.